[all-commits] [llvm/llvm-project] d618a1: [libc++] Improve LIBCXX_ENABLE_INCOMPLETE_FEATURES.

[mordante via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: d618a1cc5c3919d29ee27fa48de80bed4dcb3566
      https://github.com/llvm/llvm-project/commit/d618a1cc5c3919d29ee27fa48de80bed4dcb3566
  Author: Mark de Wever <koraq at xs4all.nl>
  Date:   2021-07-30 (Fri, 30 Jul 2021)

  Changed paths:
    M libcxx/include/format
    M libcxx/include/ranges

  Log Message:
  -----------
  [libc++] Improve LIBCXX_ENABLE_INCOMPLETE_FEATURES.

@tcanens pointed out the current behavior of the macro breaks the usage
pattern described in http://wg21.link/SD6
```
#  if __has_include(<optional>)
#    include <optional>
#    if __cpp_lib_optional >= 201606
#      define have_optional 1
#    endif
```

To support this usage pattern the hard errror is removed. Instead the
header includes nothing but the `<version>` header.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D107134